1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and a wafer treatment apparatus employed therefor as well as a semiconductor device and a cleaning method after formation of a gate electrode, and more particularly, it relates to a method of fabricating a semiconductor device capable of attaining high etching selectivity and a cleaning method after formation of a gate electrode, a wafer treatment method employed for the fabrication method or the cleaning method and a semiconductor device obtained by the fabrication method.
2. Description of the Prior Art
High performance is required particularly in a transistor employed for a logic circuit or a system LSI (large-scale integrated circuit) among semiconductor devices. In order to satisfy this requirement, the thickness of a gate insulation film of the transistor is set to not more than 3 nm. Further, a development has recently been made for reducing the thickness of the gate insulating film below 2 nm.
In etching for forming a gate electrode of the transistor, a conductive layer for defining the gate electrode is substantially etched under a condition having high selectivity for the gate insulator film, thereby preventing the thin gate insulator film from etching.
In the etching for forming the gate electrode, a reaction product resulting from the etching adheres to the surfaces of the side walls of the gate electrode and the surface of a mask member for patterning the gate electrode. It has been recognized by observation with an SEM (scanning electron microscope) that such a reaction product adheres to the surfaces substantially in a conformal state. In order to ensure reliability of a semiconductor device, such a reaction product adhering to the surface of the gate electrode must be removed.
In order to remove such a reaction product, wet cleaning employing a chemical solution is generally performed. An exemplary method of fabricating a semiconductor device including this wet cleaning is now described.
First, an insulator film for defining a gate insulator film is formed on a semiconductor substrate. A polysilicon film for defining a gate electrode is formed on the insulator film. A silicon oxide film for defining a mask member for forming the gate electrode is formed on the polysilicon film. The silicon oxide film is subjected to prescribed etching, for forming the mask member.
The polysilicon film is etched through the mask member in an atmosphere prepared by converting a gas mixture containing Cl2 and O2 or HBr, Cl2 and O2, for example, into a plasma, patterning the gate electrode. In this patterning, a reaction product adheres to the side wall surfaces of the gate electrode and the surface of the mask member. After formation of the gate electrode, the reaction product adhering to the gate electrode is removed by wet cleaning.
It is known that the reaction product is mainly composed of a silicon oxide such as SiOxCly or SiOxBry when the polysilicon film is etched by converting the gas mixture containing Cl2 and O2 or HBr, Cl2, and O2 to a plasma.
Therefore, the reaction product is removed by dipping the semiconductor substrate in a cleaning solution of diluted hydrofluoric acid (DHF) or aqueous ammonia peroxyhydrate (NH4OH+H2O2+H2O:APM), for example. Thus, the reaction product is removed for forming the gate electrode.
After the reaction product is removed, the mask member remaining on the gate electrode must be removed in order to electrically connect the gate electrode with a wire through a tungsten plug embedded in a contact hole, for example.
This mask member is formed by a silicon oxide film such as a TEOS (tetraethyl orthosilicate glass) oxide film. Therefore, the mask member consisting of the silicon oxide film is removed by diluted hydrofluoric acid, for example. Thus, the silicon oxide film serving as the mask member is removed for forming the gate electrode.
However, the conventional method of fabricating a semiconductor device has the following problems:
As hereinabove described, the reaction product adhering to the surfaces of the side walls of the gate electrode and the like when forming the gate electrode is mainly composed of a silicon oxide such as SiOxCly or SiOxBry. The gate insulator film also consists of a silicon oxide obtained by oxidizing the silicon substrate.
When the reaction product is removed by the cleaning solution of diluted hydrofluoric acid (DHF) or aqueous ammonia peroxyhydrate (APM), therefore, the gate insulator film is also etched.
Therefore, a gate insulator film 102 located between a silicon substrate 101 and a gate electrode 103 may be partially etched to expose a corner portion of the gate electrode 103, as shown in a circle 105 in FIG. 27, for example.
Therefore, a current leaks from the exposed corner portion of the gate electrode 103 to deteriorate electric characteristics of the transistor, disadvantageously resulting in reduction of reliability of the semiconductor device.
When a reaction product is removed with aqueous ammonia peroxyhydrate (APM) in a gate electrode 103 having a polycide structure formed by a polysilicon film 103a and a tungsten silicide film 103b as shown in FIG. 28, for example, a side wall portion of the tungsten silicide film 103b may be also etched (side-etched) in addition to a gate insulator film 102 (a portion in a circle 105).
In this case, the etched portion may not be fully filled but produce a void when the gate electrode 103 is covered with an interlayer isolation film, and reduce the reliability of the semiconductor device.
Further, the silicon oxide film serving as the mask member is removed by diluted hydrofluoric acid, as hereinabove described. However, the gate insulator film 102, also formed by a silicon oxide film, is simultaneously etched when the mask member is removed.
Therefore, the gate insulator film 102 located between the silicon substrate 101 and the gate electrode 103 may be partially etched to expose another corner portion located under the gate electrode 103, as shown in a circle 105 in FIG. 29.
Consequently, the current leaks from the exposed corner portion 105 located under the gate electrode 103, to disadvantageously deteriorate the electric characteristics of the transistor similarly to the case of removing the reaction product.
The present invention has been proposed in order to solve the aforementioned problems, and an object thereof is to provide a method of fabricating a semiconductor device attaining high selectivity in etching. Another object of the present invention is to provide a wafer treatment apparatus employed for such a method of fabricating a semiconductor device. Still another object of the present invention is to provide a semiconductor device obtained by such a method of fabricating a semiconductor device.
A method of fabricating a semiconductor device according to a first aspect of the present invention comprises a wafer treatment step performing prescribed treatment on a first part having a prescribed etching property and a second part having an etching property different from the prescribed etching property, which are formed on a semiconductor substrate, in a chamber with gas for etching. The wafer treatment step includes an etching gas supply step of introducing the gas for etching into the chamber. Assuming that a time between introduction of the gas for etching into the chamber and starting of etching of the first part is referred to as a first starting time and a time between introduction of the gas for etching into the chamber and starting of etching of the second part is referred to as a second starting time longer than the first starting time, a time for carrying out the etching gas supply step is longer than the first starting time and shorter than the second starting time.
According to this method, the time for carrying out the etching gas supply step is longer than the first starting time and shorter than the second starting time, whereby only the first part is etched before etching of the second part is started. Consequently, the first part can be selectively etched substantially without etching the second part.
More specifically, the time difference between the first starting time and the second starting time is preferably not more than about 5 seconds.
In this case, only a reaction product can be selectively and efficiently removed without etching a gate insulator film, for example, as described later.
Preferably, the method of fabricating a semiconductor device further comprises steps of forming an insulator film on the semiconductor substrate and forming a conductive region on the insulator film, the step of forming the insulator film includes a step of forming a gate insulator film, the step of forming the conductive region includes a step of forming a gate electrode part on the gate insulator film, the first part contains a reaction product generated before forming the gate electrode part for covering the surface of the gate insulator film and the surface of the gate electrode part, the second part includes the gate insulator film, and the gas for etching includes hydrofluoric acid gas.
In this case, the reaction product adhering when forming the gate electrode part can be selectively etched substantially without etching the gate insulator film so that the reaction product can be selectively removed.
Preferably, the wafer treatment step includes an added gas supply step of introducing reaction accelerating gas for further reducing the first starting time into the chamber before the etching gas supply step.
In this case, the reaction accelerating gas reduces the first starting time, thereby increasing the time for etching the first part. Consequently, the time for the wafer treatment step can be reduced.
More preferably, the added gas supply step and the etching gas supply step are alternately carried out in the wafer treatment step.
When the added gas supply step and the etching gas supply step are alternately carried out, the first part can be reliably selectively removed while leaving the second part.
Preferably, the added gas supply step is continuously carried out also after the etching gas supply step is started in the wafer treatment step.
In this case, the etching rate in the etching gas supply step can be improved for reducing the time for the wafer treatment step.
Preferably, the wafer treatment step includes an evacuation step of evacuating the chamber, and the evacuation step is not carried out at least while the etching gas supply step is carried out.
In this case, the internal pressure of the chamber is increased in the etching gas supply step as compared with the case of regularly carrying out the evacuation step, so that the first part can be effectively etched.
Preferably, the method of fabricating a semiconductor device further comprises steps of forming a conductive layer on the semiconductor substrate through a gate insulator film, forming a layer for defining a mask on the conductive layer, etching the conductive layer through a mask of the layer for defining a mask thereby forming a gate electrode and removing the layer for defining a mask remaining on the gate electrode after formation of the gate electrode, while the wafer treatment step includes a step of removing the layer for defining a mask, the first part includes the layer for defining a mask, the second part includes the gate insulator film, and hydrofluoric acid gas is supplied as etching gas in the etching gas supply step.
In this case, the layer for defining a mask can be selectively etched substantially without etching the gate insulator film when removing the layer for defining a mask employed for patterning the gate electrode, for selectively removing the layer for defining a mask.
Preferably, the etching gas supply step is repetitively carried out in the wafer treatment step.
If the layer for defining a mask cannot be removed by single etching, therefore, the layer for defining a mask can be reliably removed without etching the gate insulator film by repeating the etching gas supply step.
More preferably, the wafer treatment step includes an evacuation step evacuating the chamber, and the etching gas supply step and the evacuation step are alternately carried out.
Thus, the pressure in the chamber is increased in the etching gas supply step, so that the layer for defining a mask can be effectively etched.
A semiconductor device according to a second aspect of the present invention is fabricated by the method of fabricating a semiconductor device according to the first aspect.
According to this semiconductor device, the first part is selectively etched with respect to the second part, whereby a reaction product adhering when forming the gate electrode, for example, can be removed substantially without etching the gate insulator film. Further, the layer for defining a mask for forming the gate electrode can be removed substantially without etching the gate insulator film. Consequently, the semiconductor device can be prevented from deterioration of electric characteristics.
A wafer treatment apparatus according to a third aspect of the present invention is employed for performing prescribed treatment on a first part having a prescribed etching property and a second part having an etching property different from said prescribed etching property, which are formed on a wafer, with gas for etching, and comprises a chamber, an etching gas supply part and a control part. The chamber stores the wafer. The etching gas supply part supplies the gas for etching into the chamber. The control part controls supply of the gas for etching from the etching gas supply part into the chamber. Assuming that a time between introduction of the gas for etching into the chamber and starting of etching of the first part is referred to as a first starting time and a time between introduction of the gas for etching into the chamber and starting of etching of the second part is referred to as a second starting time longer than the first starting time, the control part has a function of supplying the gas for etching from the etching gas supply part into the chamber by a time longer than the first starting time and shorter than the second starting time.
According to this wafer treatment apparatus, only the first part is etched before etching of the second part is started so that the first part can be selectively etched substantially without etching the second part. Thus, only a reaction product adhering when forming a gate electrode of a semiconductor device, for example, can be selectively removed by etching substantially without etching a gate insulator film, for preventing the semiconductor device from deterioration of electric characteristics.
Particularly when the time difference between the first starting time and the second starting time is not more than about 5 seconds, the first part can be reliably selectively etched in the wafer treatment apparatus.
Preferably, the wafer treatment apparatus further comprises an added gas supply part supplying reaction accelerating gas for reducing the first starting time into the chamber, and the control part includes a function of supplying the reaction accelerating gas from the added gas supply part into the chamber before supplying the gas for etching.
In this case, the reaction gas accelerating gas reduces the first starting time, thereby increasing the time for etching the first part. Consequently, the time for treatment is reduced and the throughput of the wafer treatment can be improved.
Preferably, the control part includes a function of alternately supplying the gas for etching and the reaction accelerating gas.
In this case, supply of the gas for etching and supply of the reaction accelerating gas are so alternately carried out that the first part is repetitively etched and can be reliably selectively removed while leaving the second part.
Further preferably, the control part includes a function of supplying the reaction accelerating gas also while supplying the gas for etching.
In this case, the reaction accelerating gas is supplied while the gas for etching is supplied, whereby the etching rate for etching the first part can be improved. Thus, the time for the treatment is reduced and the throughput of the wafer treatment apparatus can be improved.
Preferably, the wafer treatment apparatus further comprises an evacuation part evacuating the chamber, and the control part includes a function of not operating the evacuation part at least while supplying the etching gas.
In this case, the internal pressure of the chamber supplied with the etching gas is increased as compared with the case of regularly evacuating the chamber, so that the first part can be effectively etched.
A cleaning method after formation of a gate electrode according to a fourth aspect of the present invention removes a reaction product formed by etching with hydrofluoric acid gas after forming a gate electrode patterned by etching with a mask on a semiconductor substrate through a gate insulating film.
According to this cleaning method after formation of a gate electrode, the reaction product can be removed without scraping the gate insulator film.
Preferably, the gate electrode consists of a film containing at least silicon.
In this case, a reaction product resulting from formation of the gate electrode consisting of the film containing silicon is mainly composed of a silicon oxide, and can be reliably removed with the hydrofluoric acid gas without removing the gate insulator film.
Preferably, the time for removing the reaction product with the hydrofluoric acid gas is within the reaction time difference between a time when the reaction product is scraped by the hydrofluoric acid gas and a time when the gate insulator film is scraped.
In this case, only the reaction product can be removed without scraping the gate insulator film.
Further preferably, the reaction time difference is repetitively set thereby removing the reaction product with the hydrofluoric acid gas.
In this case, only the reaction product can be reliably removed. More specifically, the semiconductor substrate formed with the gate electrode is set in a chamber, and the reaction time difference is repetitively set by repeating steps of evacuating the chamber and charging the chamber with the hydrofluoric acid gas.
Preferably, a set temperature for removing the reaction product with the hydrofluoric acid gas is set to a level lower than 40xc2x0 C.
In this case, reduction of the reaction time difference is so suppressed that only the reaction product can be relatively readily removed. The lower limit of the aforementioned set temperature is preferably about the room temperature.